Engraving process



Patented Oct. 31, 1939 ENGRAVING "PROCESS Edward A. Sikorski, Parma, Ohio, assignor to General Chemical Company, New York, N. Y.,

a corporation of New York No Drawing. Application May 28, 1938,

Serial No. 210,775

comm (01. 41 42) This invention relates to the art of engraving or etching metals. More particularly, the invention is directed to improvements in processes for etching metals and'in metal etching baths.

5 The general procedure involved in the engraving or etching of metals such as steel, copper and zinc is well known. Briefly, the metal to be etched is covered with a ground of acid-resistant material such as varnish, wax or resin.-

The design to be etched is made on the surface of the metal by cutting away or otherwise removing part of the ground. The metal is then subjected to the action oi an acid bath which bites out exposed or uncovered parts of the metal. The bath usually employed is a water solution containing around 10% nitric acid.

Several suggestions have been made as to addition to the nitric acid of small amounts of other substances for the purpose of improving the properties of the nitric acid principally with respect to bite, non-discoloration of the bath and of the'work, and to impart to the bath a longer life. While some progress has been made, the materials previously suggested have been such as to bring about improvements with respect to one or another but not all of the important properties a satisfactory etching process and etching bath should have. Another difilculty is that in many instances prior proposals have required 80 introduction into the nitric acid of materials which raise the impurity content of the acid above the amounts permissible in recognized trade specifications. 4

I have found that the etching process may be 85 made much more efllcient it carried out in the presence of a small amount of urea and I havediscovered that addition ot a small amount 0! urea to the nitric acid substantially enhances the 'valueof the acid with respect to each of the 40 desirable properties an etching bath should possess. While lesser amounts may'be used, more specifically I have found that much improved results may be obtained by carrying out the etchingp'rocess in'a bathcontaining nitric acid and urea, the acid and urea being inthe proportion of not less than 0.3 part by weight of urea per 100 parts by weight of nitric acid; 100% basis. I prefer to use nitric acid and urea in proportion of about 0.32 part by weight of urea per 100 parts by weight of 100% nitric acid. Greater amounts.

' oi urea may be employd to no disadvantage.

' As stated, the nitric acid concentration 0! most customary commercial etching baths is about 10%. I! desired the urea in proper amount may I; be added to the relatively dilute etching bath. However, it ismore convenient and usually more desirable to first incorporate the urea with the stronger nitric acid usually available on the market in concentrations ranging from 38 to 42 B. In practicing the invention, the strong nitric 5 acid containing the indicated amounts of urea is diluted with water in quantity to form a bath having the customary 910% or other suitable nitric acid concentration. In some types of oper-'- ation, the bath is much stronger, and in some 10 particular instances the BNO; concentration of the bath may be as highas 30-32" B. or more, dependingupon the action desired.

The bath is fed into an open rocker, splashertype or other common etchingmachine. The 15 metal to be etched is then subjected to the action of the bath for example by immersion or splashing. The technique employed is variable depending upon the type of work, depth of cut desired, time of treatment, and other factors and is well 20. understood in the etching art. The temperature of the bath is preferably kept at about F., and usually not permitted to exceed about F.

I have found that by carrying out the process as described discoloration of the work is pre- 25 vented. The'bath remains substantially water white or very slightly yellow. Little fuming of nitrogen oxides occurs. The work is clean-cut, and etched lines and dots aresharp. Addition of a small amount of urea to the acid substan- 80 tially lengthens the efiective life of the bath.

centration of the acidftemperatures, etc. The 40 'reaction of nitric acid with metals is oomphcated, usually slow at first and increasing in speed with accumulation of reduction products. The chemical reactions taking place in etching operations are dependent upon many difierent factors, 43 are very complex and verylittle is definitelyknown oi the exact nature of such reactions. 'In all etching processes there seems to be a tendency toward formation of free oxides of nitrogen in- V cluding the brown N02. In the process 01' the 80 invention, with urea present it appears that the oxides of nitrogen such as NO and NO: are largely acted upon by urea in some complex way to form gaseous nitrogen and water and possibly some NH). It definitely appears, however, that the II process of urea inhibits production of fumeiorlning nitrogen oiddes, and such inhibiting action evidently contributes in some way or other, possibly catalytically, to increase the bite of the acid and extend the life 01' the bath.

I claim:

1.' The process for etching metal which comprises subjecting the metal to the action of an etching bath comprising nitric acid having incorporated therewith a small amount of urea.

2. The process for etching metal which comprises subjecting the metal to the action of an etching bath comprising nitric acid having incorporated therewith urea in the proportion of not less than 0.3 part by weight of'urea per 100 parts by weight of nitric acid, 100% basis.

3. The process for etching metal which comprises subjecting the metal to the action of an etching bath comprising nitric acid having incorporated therewith urea in the proportion of about 0.32 part by weight of urea per 100 parts by weight of nitric acid, 100% basis.

4. An etching bath comprising nitric acid having incorporated therewithv a small amount of urea. I

5. An etching bath comprising nitric acid having incorporated therewith urea in the proportion 10 of not less than 0.3 part by weight of urea per 100 parts by weight of nitric acid, 100% basis.

6. An etching bath comprising nitric acid having incorporated therewith urea in the proportion of about 0.32 part by weight of urea per 100 5 parts by weight of nitric acid, 100% basis.

EDWARD A. SIKORSKL 

